In many manufacturing processes, such as those producing printed circuit boards and integrated circuits, it is necessary to initially produce a film from which the manufactured product is made. Photoplotters provide a means by which such films may be made directly under computer control. In the most common photoplotter configuration, a light source is moved back and forth along a fixed path above a photosensitive media while the media is moved in a second path direction. Generally, the directions of media movement and light source movement are perpendicular to each other. Under the control of the driving computer, the movements of the media and the light source are coordinated to produce latent images upon the photosensitive media. Once the latent images are exposed upon the media, more or less conventional photographic development techniques are utilized in producing the desired positive or negative film.
Because of their capability of producing film directly which is suitable for incorporation in other photographic and photolithographic processes, such as those utilized in fabricating integrated circuits, photoplotters have become an extremely important tool in many manufacturing processes. With the development and availability of computer systems having increased power and capability, the sophistication and capability of the photoplotters has also enjoyed a substantial increase. For example, certain types of photoplotters provide a variable aperture controlling the light source and facilitating the exposure of different sized image lines and objects upon the media.
While the structure of photoplotters varies substantially with designers choice, all generally combine the light source with an optics system generally comprising a plurality of lenses arranged to focus the light provided by the light source upon the media. In some systems, a fixed single aperture is interposed between the light source and the optic system to provide a single image exposing beam size. In other systems, a plurality of apertures are interchangeably supported between the light source and the optic system. In still other types of devices, the aperture is adjustable and mechanical means are provided for carrying out the aperture adjustment. In addition to the foregoing, the majority, if not all, photoplotter systems, utilize a shutter interposed between the light source and the media to permit the complete interruption of image exposing light. This feature is necessary in order to permit movement of the light source with respect to the media in a non-imaging manner between portions of the imaged pattern.
With the continued motivation to improve the photoplotter structures, practitioners in the art have attempted to refine and enhance its performance to take better advantage of the increased power of the computer systems which drive the photoplotter. However, to date, the efforts to refine the photoplotter device have met with only limited success. While several problems arise in optimizing the design of the photoplotter, one of the more serious limitations concerns the light source used to expose the media. In one type of system a flash lamp or strobed light source is used which provides sufficient power buy is subject to prohibitively long recharge times. Conventional incandescent lamps also provide considerable output power but change illumination level slowly in response to changes of applied electrical energy. Similarly, the recently developed Quartz Hallogen lamps provide efficient power sources and more than enough light energy but are similarly limited in their speed of response to changes of illuminating power. As a result, photoplotter systems using such light sources have required the implementation of complex and relatively expensive mechanical devices to provide workable photoplotters. For example, the above-described shutter is not required if the light source operating within the photoplotter may be extinguished quickly once the energy to the light source is cut off. Similarly, it is well within the capability of modern computer systems used to drive photoplotters to improve system imaging by modulating the intensity of the light source in coordination and synchronization with the movement of the photoplotter head an the media. However, once again, because conventional light sources do not respond quickly to changes in applied power, such modulation of light intensity can only be carried out effectively at extremely slow plotter speeds.
Current laser plotters generally modulate the light beam with an acousto-optic modulator or electro-optic modulator controller by electric power. While such lasers provide high speed, the combination of laser and modulator is expensive. In addition, laser systems require polarizers and precise alignment with the laser beam.
There remains therefore, a need in the art for an improved photoplotter imaging system in which the intensity of the light source may be controlled easily and effectively and with sufficient speed to optimize the operation of the photoplotter.